1. Field Of The Invention
The invention relates to apparatus for intravascular imaging structures and in particular to mechanical guided directional coronary atherectomy catheters.
2. Description Of The Related Art
Ultrasonic imaging catheters, such as the one set forth in U.S. Pat. 5,193,546, which is incorporated herein by reference, are well known and have been developed to provide cross-sectional structural images of blood vessels and their lumens such as arteries in the vicinity of the heart. Operation of mechanically driven types of imaging catheters involves the insertion of a protective sheath which surrounds a long thin rotatable cable assembly with a transducer subassembly attached to its distal end into the blood vessel of interest. This complete assembly is the imaging catheter. The operator positions the contained transducer subassembly at a location within the blood vessel near the structures to be imaged as is illustrated in the aforementioned patent.
Ultrasonic images of the inside of the blood vessel are formed by using a sonar-like technique. In such assemblies, the transducer subassembly includes a transducer element rotatably mounted within the subassembly, generating a series of pulses which are transmitted outward from the transducer as the transducer is moved through 360.degree. of rotation. The transmitted and echo ultrasonic pulses are substantially able to pass through the material of the sheath. Echo pulses reflected from structures inside the blood vessel lumen and the wall are received between transmitted pulses by the transducer and collected by control apparatus coupled with the rotating transducer and cable assembly within the sheath and displayed as a cross-sectional ultrasonic image of the inside of the blood vessel as set forth in U.S. Pat. 4,917,097, which is incorporated herein by reference.
A directional atherectomy catheter, such as is set forth in U.S. Pat. 4,794,931, which is incorporated herein by reference, is structurally similar to the imaging catheter. A cylindrical cutter is attached to the distal end of a long thin rotatable cable assembly which is contained within a sheath. Attached to the distal end of the sheath is a metallic cylindrical housing which surrounds and contains the cutter. The housing has a cut-out section, hereinafter referred to as the window, which exposes a cutting edge of the contained cutter. Opposite the window, a balloon is mounted on the housing. In use, a physician or operator uses fluoroscopy to position and direct the housing window toward an atheroma blocking blood flow in the lumen of the blood vessel. The balloon is inflated to press the housing window against the atheroma. The cutter is mechanically rotated via the long thin cable assembly and advanced distally through the housing to cut any atheroma pressed inside the housing window and pushes the cut atheroma, for storage, into a nosecone mounted on the distal end of the housing. An atherectomy procedure may require many cuts, controlled balloon inflations/deflations and positionings of the housing and window to remove and collect the desired amount of atheroma.
Guided directional atherectomy adds a transducer in, or near, the cutter to give the directional atherectomy catheter the ability to ultrasonically image the blood vessel. The ability to image the blood vessel allows a more precise and efficient catheter positioning and cutting of the atheroma. The housing is generally constructed of a metallic material which will not substantially pass ultrasonic pulses. Therefore, the guided directional atherectomy catheter may only be used to image through the housing window. Generally, the window is cut in the housing surface at an angle of approximately 120.degree. with respect to a center axis of the housing. Therefore, only a 120.degree. section image view of the blood vessel adjacent to the housing window is obtained.
In a mechanically operated imaging or guided directional atherectomy catheter assembly, the transducer and cutter are located at the distal end of a long thin cable assembly generally comprised of a duplex spring assembly surrounding a coaxial cable extended through the sheath and connected at the proximal end to the control apparatus. The control apparatus rotates the proximal end of the cable assembly at a constant rate, typically 1800 RPM, and causes the transducer to generate an ultrasonic pulse at regular intervals, for example, approximately every 1.4.degree. of rotation of the proximal end of the cable assembly. Thus, in the guided directional atherectomy catheter, approximately 85 transmit/receive cycles are generated as the transducer is transmitting pulses out of the housing window to create the 120.degree. section image view of the blood vessel adjacent the housing window. The control apparatus operator can rotate the 120.degree. housing window to view the entire inner surface of the blood vessel and move the transducer and cutter distally through the area of the housing window so as to remove the atheroma forced through the window by the catheter balloon.
A problem occurs in that the operator located at the proximal end of the catheter cable assembly is unaware of the precise position of the transducer and cutter within the catheter housing at the distal end of the cable assembly. Without the operator's knowledge, the transducer and cutter may be located at the rear, middle or front of the window area.
Accordingly, a need exists in the art for a guided directional coronary atherectomy catheter for use in determining the location and direction of travel of the to transducer and cutter in the window area of the distal ultrasound imaging catheter apparatus.